1. Field of the invention
The present invention relates to a magnetic tape recording apparatus (hereinafter referred to as VTR) and particularly to a VTR in which only the necessary amount can be recorded intermittently as required and recording can be made for 24 hours or more by using one cassette.
2. Description of the Prior Art
FIG. 1 is a perspective view showing a tape running mechanism of a conventional VTR of a helical scanning system and FIGS. 2 and 3 are plan views thereof. FIG. 4 is an illustration showing an example of a magnetic tape pattern recorded by a conventional VTR.
First, referring to FIGS. 1 through 4, a conventional VTR of a helical scanning system will be briefly described. A magnetic tape 20 wound on a supply reel 11 of a cassette 10 is drawn out from the cassette 10 by means of guide rollers 31 and 32 on the ingoing side and a guide roller 33 on the outgoing side. Then, the magnetic tape 20 is brought into contact with a total width erasing head 41 and an impedance roller 51 and after that it is wound on a rotating drum 60. Further, the magnetic tape 20 is brought into contact with an impedance roller 52, a sound erasing head 42 and a sound control head 43, and then it is held between a capstan shaft 72 and a pinch roller 71. Thus, the magnetic tape 20 is held under pressure between the capstan shaft 72 and the pinch roller 71 and when the capstan shaft 72 rotates, the magnetic tape 20 can be made to run for recording or reproduction so as to be wound on a take-up reel 12 in the cassette 10.
The above stated rotating drum 60 comprises an upper rotating drum 61 and a lower fixed drum 62 as a pair and by means of a video head 63 contained in the upper rotating drum 61, a video signal is recorded on video tracks 21 and 22 of channels 1 and 2 of the magnetic tape 20 as shown in FIG. 5. In addition, a sound signal is recorded on audio tracks 23 and 24 of the channels 1 and 2 of the magnetic tape 20 by means of a sound recording portion of the sound control head 43. The total width erasing head 41 erases all the signals recorded till then in the magnetic tape 20, that is, the video signal, the sound signal, the control signal and the like and the sound erasing head 42 serves for so-called after recording.
Thus, the magnetic tape 20 drawn out from the cassette 10 is correctly brought into contact with the respective heads whereby the respective signals can be correctly recorded in or reproduced from the magnetic tape 20 as shown in FIG. 4.
In such a conventional VTR as described above, the above described operation involves no problem if recording is made continuously. However, continuous recording by such a conventional VTR can be made only for several hours by using one cassette.
On the other hand, a monitoring VTR is widely utilized in a bank, a store etc. Generally, such a VTR for monitoring needs only to make recording in a period when an event occurs, for example, in a period when a human enters a store. Accordingly, for the purpose of making recording for a long period, such a conventional monitoring VTR is brought into a pause state when recording is not required. More specifically, the conventional VTR is brought into the state shown in FIG. 3 in which the magnetic tape 20 is made to stop running by stopping only the rotation of the capstan shaft 72. In this case, however, the rotating drum 60 still rotates in this pause period by 1800 rpm for example and if this state is continued for a long period, it is feared that the video head 63 might be worn out or the magnetic tape 20 might be damaged by the video head 63 if a large tension is applied to the magnetic tape 20. For this reason, a pause state or a reproduced still state is always made to be automatically cancelled after several minutes.
In addition, in order to dissolve the above described drawbacks, a method might be used in which the loaded state of the tape as shown in FIG. 2 (an enabled state for recording) is cancelled and the VTR is made on standby in an unloaded state as shown in FIG. 3 and is brought into the loaded state when an event occurs. In this method, however, the guide rollers 31 and 32 on the ingoing side and the guide roller 33 on the outgoing side are moved each time from the state shown in FIG. 2 to the state shown in FIG. 3, in other words, they are moved for loading the tape as shown in the figure by the arrows, which causes wearing out of the loading mechanism for driving the above described guide rollers 31, 32 and 33. Furthermore, a large driving force is generally required for the loading mechanism and accordingly a noise occurs in loading, which is not desirable considering the purpose of monitoring. Moreover, since loading of the tape is started after an event occurs, a lapse of several seconds is unavoidably required as the time for loading the tape and as a result responsitivity for monitoring is not good.